1. Field of the Invention
This invention relates to an improvement in methods of copolymerizing ethylene with other olefins and, in particular, this invention relates to an improved method of copolymerizing ethylene and another olefin to obtain linear low density polyethylene copolymers.
2. Description of the Prior Art
U.S. Pat. No. 3,087,917 issued Apr. 30, 1963 in the name of J.S. Scoggin discloses a process of polymerizing ethylene in the presence of a liquid reaction medium and a catalyst to produce polymers in the form of solid particles which are dispersed in the liquid reaction medium as they are formed. This type of reaction method is known to those skilled in the art as a "slurry" polymerization process and is widely used for the production of high density polyethylene.
Such slurry processes conventionally use isobutane, pentane, hexane or a similar saturated aliphatic hydrocarbon as the liquid reaction medium (also referred to as the "diluent"). However, when applied to the production of ethylene/olefin copolymers, and especially the type of copolymer known as "linear low density polyethylene (LLDPE)", conventional slurry processes, including the so-called "particle form" process, exhibit certain disadvantages. For example, the operable reaction temperature is limited because, at high temperatures, linear low density polyethylene goes into solution, and heat transfer and production rates are adversely affected. Furthermore, complete separation of the comonomer and the reaction medium is difficult, and product copolymers generally exhibit relatively broad molecular weight distributions and contain undesirably high amounts of wax. The bulk density of copolymers produced by prior slurry processes tends to be low.